Kaṭha Upaniṣad 258

258

tām īdṛśīṃ tadavasthāṃ yogam iti manyante viyogam eva santam / sarvānarthasaṃyogaviyogalakṣaṇā hīyam avasthā yoginaḥ / etasyāṃ hy avasthāyām avidyādhyāropaṇavarjitasvarūpapratiṣṭha ātmā / sthirām indrayadhāraṇāṃ sthirām acalām indrayadhāraṇāṃ bāhyāntaḥkaraṇānāṃ dhāraṇam ity arthaḥ / apramattaḥ pramādavarjitaḥ samādhānaṃ prati nityaṃ yatnavāṃs tadā tasmin kāle, yadaiva pravṛttayogo bhavatīti sāmarthyād avagamyate / na hi buddhyādiceṣṭābhāve pramādasaṃbhavo 'sti / tasmāt prāg eva buddhyādiceṣṭoparamād apramādo vidhīyate / athavā yadaivendriyāṇāṃ sthirā dhāraṇā, tadānīm eva niraṅkuśam apramattatvam ity ato 'bhidhīyate 'pramattas tadā bhavatīti / kutaḥ / yogo hi yasmāt prabhavāpyayāv upajanāpāyadharmaka ity arthaḥ / ato 'pāyaparihārāyāpramādaḥ kartavya ity abhiprāyaḥ ///2,3.11 //

This firm holding back of the senses is what is known as Yoga. Then one should become watchful, for Yoga comes and goes.